The high cost of modern harvesting machines, such as combine harvesters or self-propelled forage harvesters, urges the to employ their machines in a broader range of crop types, geographical locations and weather conditions. Manufacturers, on the other hand, make their machines more versatile through the addition of net crop processing features and the widening of the ranges of the known machine settings. Another trend is the increased machine capacity which reduces the necessary harvesting time per hectare of crop.
As a result both quantity and frequency of machine adjustments have increased substantially. For ease of operation, almost every setting is controlled by the operator from his driving position on the steering platform. The original, fully mechanical control has been replaced by electrical control systems, which may incorporate an electronic device of automatically changing the machines settings, thus simplifying the tasks of the operator.
Actual electrical control systems on harvesting machines require a large number of wires and connections, because a separate conductor is needed for every signal from and to the operator's position. Reliability of such systems is hard to maintain, because chances for mistakes during manufacturing, such as loose or switched connections, are great and the physical connections at the end of the wires are subject to mechanical vibrations and atmospheric corrosion. Moreover, long wires are subject to electromagnetic interferences by radio transmissions and feed unwanted signals to the operator's platform.
If one wants to add a new control device to the harvesting machine, he will be forced to lead new wires to the operator's platform. Therefore, actual electrical systems cannot easily be adapted to modified features, meaning that their flexibility is limited.
The use of a single electronic device, such as a central microprocessor unit, makes the whole machine vulnerable to a complete breakdown caused by failure of this device, and requires a large number of wire connections at the location of the microprocessor unit.
A part of these problems can be overcome by using a plurality of microprocessor units which enable a decentralization of the machine controls. In such configuration every microprocessor handles sensor signals from and actuator signals to a limited part of the machine. This reduces the number of wired connections to the steering platform, but limits the availability of machine data: a signal from one sensor will only be fed to one microprocessor, unless additional wires feed the same signal to other units.
WO-A-82/01354 shows an embodiment in which a plurality of microprocessor units is controlled by a single master microprocessor. Data between the other, slave units have to pass through the master microprocessor, to which the first slave unit has to send a data request signal, whereupon the master microprocessor may or may not immediately request the data from a second slave unit and may deliver the obtained data to the first slave unit. Such process is cumbersome and disturbances to the master unit will affect the whole machine.